At the present time, major somatic gene transfer approaches employ either viral (Morgan, J. R., Tompkins, R. G., and Yarmuch, M. L. (1993), "Advances in recombinant retroviruses for gene delivery," Adv. Drug Del., Rev. 12, 143-158; Colledge, W. H. (1994), "Cystic fibrosis gene therapy." Curr. Opin. Gene Develop., 4, 466-471; Trapnell, B. C. and Gorziglia, M. (1994), "Gene therapy using adenoviral vectors," Curr. Opin. Biotechnol., 5, 617-625) or nonviral vectors (Cotten, M. and Wagner, E. (1993), "Non-viral approaches to gene therapy," Curr. Opin. Biotech., 4, 705-710; Ledley, F. D. (1994), "Non-viral gene therapy," Curr. Opin. Biotechnol., 5, 626-636).
Viral vector-directed gene transfer shows high gene transfer efficiency but is deficient in several areas. For example, some viral vectors randomly integrate DNA into host genomes (Olsen, J. C., Huang, W., Johnson, L. G., and Boucher, R. C. (1994) "Persistence of adenoviral vector gene expression in CF airway cells is due to integration of vector sequences into chromosomal DNA," Pediatr. Pulm. S10, 230; Russel, D. W., Miller, A. D., and Alexander, I. E. (1994) "Adeno-associated virus vectors preferentially transduce cells in S phase," Proc. Natl. Acad. Sci. 91, 8915-8919) posing potential risks, including neoplastic transformation (Colledge et al. (1994) supra; Fairbairn, L. J., Cross, M. A. and Arrand, J. R. (1994) "Paterson Symposium 1993-Gene therapy," Brit. J. Cancer, 59, 972-975). In addition, adenoviral vectors induce host inflammatory and immune responses, rendering these vectors ineffective in repeated application (Ginsburg, H. S., Moldawer, L. L., Schgal, P. B., Redimgton, M., Kilian, D. L., Chanock, R. M., and Prince, G. A. (1991), "A mouse model for investigating the molecular pathogenesis of adenovirus pneumonia," Proc. Natl. Acad. Sci. USA 88, 1651-1655; Yang, Y. P., Nunes, F. A., Berencsi, K., Gonczol, E., Engelhardt, J. F., and Wilson, J. M. (1994), "Inactivation of E2a in recombinant adenovirus improves the prospect for gene therapy in cystic fibrosis," Nature Genetics 7, 362-369; Yei, S., Mittereder, N., Tany, K., O'Sullivan, C., and Trapnell, B. C. (1994) "Adenovirus-mediated gene transfer for cystic fibrosis: Quantitative evaluation of repeated in vivo vector administration to the lung," Gene Therapy 1, 192-200; Trapnell et al. (1994), supra). Retroviral vectors require dividing cells for stable integration (Miller, D. C., Adam, M. A., and Miller, A. D. (1990), "Gene transfer by retrovirus vectors occurs only in cells that are actively replicating at the time of infection," Mol. Cell Biol. 10, 4239-4242), making these vectors unsuitable for gene therapy of terminally differentiated cells. Similarly, for efficient transduction, adeno-associated virus prefers cells in the S phase to the cells in stationary culture (Russel et al. (1994) supra). Furthermore, the requirement of adenovirus and high multiplicity of infection of adeno-associated virus for efficient transduction (Russel et al., (1994) supra) coupled with difficulty in obtaining virus preparations with high titer has limited the use of this virus as a routine gene therapy vector.
Some of the problems associated with using these viral vectors can be circumvented using gene transfer agents, such as molecular conjugates (Wu, G. Y. and Wu, C. H. (1987) "Receptor-mediated in vitro gene transformation by a soluble DNA carrier system," J. Biol. Chem. 262, 4429-4432; Wagner, E., Zenke, M., Cotten, M., Beug, H., and Birnstiel, M. L., (1990) "Transferrin-polycation conjugates as carriers for DNA uptake into cells," Proc. Natl. Acad. Sci., USA 87, 3410-3414; Findeis, M. A., Merwin, J. J., Spitalny, G. L., and Chiou, H. C. (1993), "Targeted delivery of DNA for gene therapy via receptors," TIBTECH 11, 202-205; Ferkol, T., Kaetzel, C. S., and Davis, P. B. (1993), "Gene transfer into respiratory epithelial cells by targeting the polymeric immunoglobulin receptor," J. Clin. Invest. 92, 2394-2400; Monsigny, M., Roche, A.-C., Midous, P., and Mayer, R. (1994) "Glycoconjugates as carriers for specific delivery of therapeutic drugs and genes," Adv. Drug Del. Rev. 4, 1-24; Yin, W. and Cheng, P-W. (1994), "Lectin conjugate-directed gene transfer to airway epithelial cells," Biochem. Biophys. Res. Commun., 205, 826-833) and cationic liposomes (Felgner, J. H., Gadek, T. R., Holm, M., Roman, R., Chan, H. W., Wenz, M., Northro, J. P., Ringold, G. M., and Danielsen, M. (1987) "Lipofectin: A highly efficient, lipid-mediated DNA-transfection procedure," Proc. Natl. Acad. Sci., USA 84: 7413-7417). Molecular conjugates are prepared by chemically linking receptor ligands with polycations (Wagner, E., Curiel, D., and Cotten, M. (1994), "Delivery of drugs, proteins and genes into cells using transferrin as a ligand for receptor-mediated endocytosis," Adv. Drug Del., Rev. 14, 113-135). Molecular conjugates for receptor-mediated gene delivery can also be prepared by chemically linking antibodies or fragments thereof with polycations (Cotten, M. and Wagner, E. (1993) supra; references therein and Ferkol, T. et al. (1993) supra). The polycations serve as carriers of DNA while the ligands target the receptors on cell surfaces. Upon binding to the receptors, the conjugates along with the DNA are internalized via receptor-mediated endocytosis (Findeis (1993) supra; Wagner (1994) supra; Curiel, D. T. (1994), "High-efficiency gene transfer employing adenovirus-polylysine-DNA complex," Nat. Immun., 13, 141-164).
In cationic liposome-mediated gene transfer, liposomes bind to DNA via ionic interaction and liposomes facilitate the delivery of DNA presumably by fusion with the plasma membrane (Felgner et al. (1987) supra) and/or endocytosis (Zhou, X. and Huang, L., (1994) "DNA transfection mediated by cationic liposomes containing lipopolylysine: characterization and mechanism of action," Biochem. Biophys. Acta., 1189, 195-203. These agents are easy to prepare, can deliver DNA of any size (Wagner et al. (1994) supra), but generally suffer from low transfection efficiency (Colledge (1994) supra).